I want to join the army reserves as a 35L, how do I know if it’s the right mos for me and is it hard?

If you fail, chances are they will make you a 35F. I generally think there are three classifications of "HARD" in the military.
1) Physical - this ranges from Basic Training Hard all the way to SF selection hard.
2) Technical/Academic - this ranges from Food Prep all the way to Turbine Engine Repair.
3) Mental - this ranges from HR specialist all the way to criminal investigations.

You as an infantryman have the physical part knocked out. Done. Don't worry about it.

I was a 35F, and it was the easiest job I've ever had in my entire life. I have academic writing down pat, and PowerPoint presentations as part of my life all before I even thought of the military. But I was 39 with an MBA. Class peers were teenagers that just graduated high school and were struggling with typing speed and desk top office software.

35M and 35L are among the more MENTAL MOS's out there (as observed from my unit peers in an MI BN). They have live scenarios (much like a police academy I graduated). So there are mind games being played, and this is where folks screw up. You're not always being tested on multiple choice questions. You are being tested on your poise, your interaction with suspects/witnesses, and there is thinking on your feet based on the fluid nature of the role players.

That's where it's going to be "hard" if it were hard, but hey, if you were a 20 year veteran cop it would be a piece of cake (like it was for me to sleep walk through 35F school).

Now, always remember, it's the military, whatever they are testing you on (live scenario or not) they have left and right limits. So long as you stay within the limits of what you are expected to display your competency on the live scenario role players will absolutely stay in the bounds of the scenario per their rules of engagement. The HARD PART for folks with weak people skills is once the students diverge from what they were instructed to do and go off the reservation it's GAME ON for the role players if they so choose.

Stick to your training.

If you're worried that it's hard, it might be too hard for you. If you fail the training you'll just be sent to a different MOS that your reserve unit needs.

How do you know if it's the right MOS? I think it's almost impossible to know what MOS is the right one until you've been in the Army a few years

The army is hard. If hard is what you fear, try the other services. But, then again, military life is hard. That is what makes it powerful.